It is a bit more nuanced than that, AFAIK they added female versions of a specific subset of human military, which for all this time were explicitly male with various lore reasons supporting that fact.

I'm not involved in the hobby myself but went on the occasional lore reading spree on the wikis, as s disclaimer. But I can understand that fans are upset about retcons such as this, where there wasn't much ambiguity in the lore to squeeze the change in, making it a pretty hard lore break

Yeah, there were a group of male supersoldiers (custodes) who were literally written originally to be a "brotherhood" in the early lore from the 90s, and always represented that way. They changed it in a lore shitty way, essentially saying there were women custodes all along and never addressing the original lore

Some folks were like "yo, write consistently, and if you wanna change something, just change it with new lore" and some folks were like "" ahhh fuck women! They could never hack it as my favorite character!!!!"

it literally doesn't matter because these supersoldiers are works of science fiction, so removed from reality that they are hand built by techno gene craft. There's nothing about the setting that would make such fiction impossible

I think a big part of the anger was about the fact that it was a hard retcon. It used to be official in the lore that "all space marines are male," but Games Workshop changed it to "there have always been female space marines".

40k drama is always more complicated than that while being way dumber at the same time

Yes, pretty much.